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Rear disc brakes...worth the effort?
#1169954 Fri Jun 17 2016 10:04 PM
Joined: May 2005
Posts: 1,633
B
beltfed Offline OP
Shop Shark
I'm running an '84 corvett front end with disc brakes. The rear brakes are '86 Camaro drums. As I have a pedal through the floor set up, it means a long pedal throw before the brakes grab. Due to limited space I run a dual booster under floor to get all the braking I can which isn't enough as I don't think I can lock up the wheels. I would like to know if you think converting the drums to disc would make that much difference in total braking?
Another question is...is there a way to change the pedal ratio? If I could figure out how to get a longer throw of the pedal below the pivot point that would allow me to decease the pedal ratio and move the brake grab point higher on the pedal. I see no way to move the pivot point up to decease the ratio. Thanks


Six volt guy living in a twelve volt world
Re: Rear disc brakes...worth the effort?
beltfed #1169956 Fri Jun 17 2016 10:17 PM
Joined: Jun 2011
Posts: 3,329
E
Shop Shark
The brakes should be able to be locked up, even with a Corvette disc/drum master w/o boost.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
Re: Rear disc brakes...worth the effort?
beltfed #1170115 Sat Jun 18 2016 03:06 PM
Joined: Feb 2000
Posts: 4,110
J
Shop Shark
I agree with Ed, you should have no problem stopping with what you have. Not seeing the set up, it's hard to tell what you have going on. Pedal ratio might be easy enough to change, but then we can see what you have now. Power and manual brakes have different ratio's, so maybe you some how got the wrong one.

Try adjusting the rear drums up tight, then see if the pedal is tight or about the same, if its tight, your self adjusters are not working or you need some residual valves in line to hold a couple pounds of pressure on the rear brakes. If the pedal is still soft, you have other problems.

Joe

Last edited by Joe H; Sun Jun 19 2016 02:45 PM.
Re: Rear disc brakes...worth the effort?
beltfed #1170134 Sat Jun 18 2016 06:49 PM
Joined: Feb 2004
Posts: 20,164
H
Boltergeist
It sounds like your master cylinder has too small a bore diameter. You need to move enough fluid to get the pads and shoes tight against the drums and rotors. Try going to a M/C about 1/16" or 1/8" larger diameter and see if you get a shorter pedal stroke.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Rear disc brakes...worth the effort?
beltfed #1170225 Sun Jun 19 2016 01:11 AM
Joined: May 2005
Posts: 1,633
B
beltfed Offline OP
Shop Shark
I'm running a Corvette master cylinder now. It's got a 1-18" bore. I did notice the last time I bled the fronts how little fluid came out per pedal stroke. My old Ford pickup with disc will shoot a stream half way across the room. Maybe I'd better start looking at the residual valve and proportioning valve to see if there is a restriction somewhere. I'll take a look at the flex lines also.
I'll just start at the master cylinder and work toward the wheels.
I know I've got the wrong pedal ratio as car started out with non booster. Pedal through the floor creates a problem that I have yet to figure out a solution to. Thanks.

Last edited by beltfed; Sun Jun 19 2016 01:19 AM.

Six volt guy living in a twelve volt world
Re: Rear disc brakes...worth the effort?
beltfed #1170301 Sun Jun 19 2016 03:29 PM
Joined: May 2005
Posts: 1,633
B
beltfed Offline OP
Shop Shark
I have been scratching my head on this issue. I'm a one man show in the shop, and getting up and down is really getting as old as I.
I purchased a cheap vacuum pump last year to get my Zephyr vacuum wipers working. It pulls a steady 15"Hg of vacuum. I'm thinking that if I use it with a catch bottle and timer I can test out the four brake lines and the other inline devices. 15"Hg of vacuum should do it, right?
On another note, a car buddy who owned two brake shops at one time thinks there are three different proportional valves. One for disc/disc, one for disc/drum and another for?. Care to comment on that idea?


Six volt guy living in a twelve volt world
Re: Rear disc brakes...worth the effort?
beltfed #1170309 Sun Jun 19 2016 04:09 PM
Joined: Feb 2004
Posts: 20,164
H
Boltergeist
One of the purposes of a proportioning valve is to restrict pressure to the front brakes to avoid front wheel lockup on slippery surfaces like mud or snow. The rear brakes only are supposed to apply with light pedal pressure so the vehicle can still be steered- - - -locked-up front wheels tend to plow ahead in whatever direction the suspension is pointed under slippery conditions. It's possible your valve is clogged, or the opening pressure is way too high to the front brakes for some reason. Try bypassing the proportioning valve temporarily and see if the brake performance improves.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Rear disc brakes...worth the effort?
beltfed #1170437 Mon Jun 20 2016 03:49 PM
Joined: Jun 2009
Posts: 1,885
4
Shop Shark
Is the Corvette master you're using meant for disc/drum or disc/disc?

If you're not getting the brakes locking up with your rear drums, discs won't help the matters. Sounds like you've got to have an expert look at your system.


1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket
Re: Rear disc brakes...worth the effort?
beltfed #1170473 Mon Jun 20 2016 09:41 PM
Joined: May 2005
Posts: 1,633
B
beltfed Offline OP
Shop Shark
The proportional valve is a PV-2 which is for disc/drum. Got the brakes bled. The vacuum pump didn't work out for the front for some reason. The front section of the master cylinder may have a blocking valve in the rest position. The proportioning valve was in a failed position because of the blown rear cylinder. I reset it per instructions.
Just used self bleeders with pumping pedal and all seems well except the long throw on the pedal as I described before. Short of a major re-design of the brake pedal setup I'll not be on anyone tail end. I really dread the rush hour traffic in Louisville at the NSRA Nations.
Thanks for the help guys!


Six volt guy living in a twelve volt world
Re: Rear disc brakes...worth the effort?
beltfed #1170483 Tue Jun 21 2016 12:11 AM
Joined: Mar 2005
Posts: 1,701
R
Shop Shark
Since you are using the under floor master and booster, do you have any residual valves in the system?

Seems in the back of my feeble mind I had to install a 2lb residual valve for the front and a 10lb for the rear (disc/drum) when I was having pedal problems on a 50 Chevy I built.

The residual valves helped a lot.

But maybe others with better ideas will chime in soon.

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